It would make a difference which area of Wales they were born and lived in.

Welsh speaking until early 20th century was positively discouraged by the government and the teaching of it in schools not allowed until about mid 20th Century they went completely the other way which I think had something to do with the rise of the Welsh Independent groups. There was even some violence.

However I would expect that it was the first language in some areas and in homes and they would have learnt it from living with large family groups through generations which carried on through the 20th century.

My boss who was younger than me spoke fluent Welsh but its because that's all they spoke in their home. She didn't learn it at school. When I went on a conference road tour with her that included a lot of Wales it fascinated me how she just naturally switched to Welsh as we crossed the Severn Bridge when speaking to her family on the mobile, although when I went for dinner they all spoke English when I was there.

I may have got my timings above wrong but maybe Guinevere can advise on this.